by erichayes » Sun Dec 03, 2006 11:34 pm
Hi All,
As I've mentioned in other threads, cathode stripping is a very real problem with output tubes, but all we were talking about was output tubes. Well, guess what . . .
It is an issue with rectifier tubes; we just don't pay that much attention to it because rectifiers either burn out or blow up before they have a chance to wear out. But when they do wear out, it's because the cathode--whether directly or indirectly heated--doesn't have sufficient emissive material left to supply the current needed by the rest of the circuit. Generally, with filament type rectifiers such as the 5U4, the filament becomes so weak physically that it sags and touches a point on the plate structure, causing a short. 5AR4s, when they live long enough, will exhibit the same shiny plating phenomenon on the inside of the envelope that output tubes show.
But what, you say, about the 12X4s in PAS preamps, or 6X5s and 5Y3s in the little integrated amps? I have a wall of Berlant Concertone record/play preamps built in the '50s and used professionally for God knows how many hours, and every one has its original Berlant branded 6X5. In record mode, total current draw is around 40 mA, which is about half of what the 6X5's capable of handling. But the more meaningful parameter is the voltage on the input cap: 330VDC.
Cathode stripping is primarily an electrostatic phenomenon, rather than thermionic. 5U4s and 5AR4s typically operate in the 450~525VDC region, where electrostatic migration of particles just starts really waking up (although corona discharge has been observed at voltages as low as 90VDC).
Bottom line: If you use tube rectifiers, get comfortable with the fact that they fail, regardless of cause, and there's not a damned thing you can do about it.
Eric in the Jefferson State